Abstract: There is a suite of volcanic-sedimentary strata comprising Dongxileke Formation in Kanasi area; nevertheless, there is no accurate ages and geochemical study for these rocks. The genesis and geodynamical background are in controversy for a long time. To tackle these problems, the authors chose a layered mylonitic dacite in this area as the study object. LA-ICP-MS zircon U-Pb ages of(445.4±3.1)Ma and(445.7±3.7)Ma from two mylonitic dacite samples suggest that these volcanic rocks erupted in late Ordovician and that Dongxileke Formation was formed in late Ordovician. The mylonitic dacite is characterized by SiO₂ content from 53.99% to 71.04%, high Al₂O₃ (13.83%-16.27%) and low MgO (1.09%-3.53%), with K₂O between 1.11% and 3.83%, 2.84% on average, suggesting high alkline volcanic rock series. The mylonitic dacite forms a LREE-enriched distribution pattern with the existence of negative Eu anomaly (δEu=0.65-0.82). Geochemical characteristics of mylonitic dacite suggest island-arc volcanic features characterized by enrichment of LILE such as Rb, Th and Ce and depletion of HFSE such as K, Nb and Ta. Combined with regional data and the results obtained by this study, the authors consider that the volcanic rocks were generated during syn-collision of the late stage subduction in an active continental margin arc. The background was the conversion from the northward subduction of Paleo-Asia Ocean under the Siberia plate to Altay micro landmass land, which resulted in the formation of late Ordovician volcanic rocks in an active continental margin arc of Kanasi area.